(1) Field of the Invention
This invention relates to the optical detection of chemical species which are dissolved in solution and, particularly, to sensor cells for use in such detection. More specifically, this invention is directed to improvements in the optical identification of ion species, molecular species, antigens, antibodies, DNA and RNA, as well as the determination of their concentrations, and especially to liquid waveguide capillary cells which, when employed as sensors, permit such improvements to be achieved. Accordingly, the general objects of the present invention are to achieve novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
There are two principal classes of chemical sensor based on the use of fiber optics. In a first of these classes of chemical sensor, sensing molecules, such as color indicating dyes, are immobilized at the distal end of a solid optical fiber. The response of the immobilized sensing molecules to an analyte in solution may be measured either by light absorption or fluorescent light spectroscopy. Sensors of this type are characterized by low sensitivity and slow response time. In order to increase the intensity of the signal of interest to a practical level, when the detection modality is absorption, a high concentration of color indicator would have to be immobilized in a very limited surface area. A high concentration of the dye, however, has a buffering effect upon the analyte thus slowing response time. In addition, loading a high concentration of dye in a small area is technically difficult because it is desirable that all of the sensing dye molecules be in contact with the fluid analyte without being leached away. If the dye capture "structure" is made very permeable to the analyte molecules to maximize such contact, then the color indicator either cannot be loaded in high concentration or it will leach out easily. Conversely, if the capture "structure" is designed to prevent indicator molecules from leaching out, normally the analyte cannot quickly penetrate such structure and react with the dye.
In the second class of sensor, known in the art as evanescent sensors, light transmitted through a solid optical fiber "leaks" to the fiber's exterior surface. The sensor normally has a single layer of sensing material applied to the exterior of the fiber optic for direct exposure to a sample solution. This single layer coating allows for intimate interaction between sensing molecules and analyte molecules in the sample solution. Evanescent sensors, while having a fast response time, are characterized by limited sensitivity.
For many applications, a sensor configured as a flow cell is preferred because a flow cell provides a sensing environment in which mechanical structure, ambient light, temperature, sample exchange rate and sample volume may be accurately controlled. For a sensor of the types described above to be installed inside a flow cell, the cell must obviously be larger than the sensor. Thus, an evanescent wave sensor would be normally inserted axially inside of flow cell defining tubing. Since evanescent wave sensors are normally composed of stripped silica fibers which are fragile, the requisite liquid tight seal between the fiber and the flow cell tubing is difficult to achieve.
A unique sensor based upon use of a liquid waveguide capillary as a flow cell, hereinafter a "LWCC" is described In U.S. Pat. No. 5,570,447. In an LWCC a capillary tube comprised of glass, quartz, silica or a suitable polymer which has been exteriorly coated with a low refractive index material acts as a fiber optic when filled with most fluids.